Photographic elements which produce images having an optical density directly related to the radiation received on exposure are said to be negative-working. A positive photographic image can be formed by producing a negative photographic image and then forming a second photographic image which is negative of the first negative--that is, a positive image. A direct-positive image is understood in photography to be a positive image that is formed without first forming a negative image. Direct-positive photography is advantageous in providing a more straight-forward approach to obtaining positive photographic images.
A conventional approach to forming direct-positive images is to use photographic elements employing internal latent image-forming silver halide grains. After imagewise exposure, the silver halide grains are developed with a surface developer--that is, one which will leave the latent image sites within the silver halide grains substantially unrevealed. Simultaneously, either by uniform light exposure or by the use of a nucleating agent, the silver halide grains are subjected to development conditions that would cause fogging of a surface latent image forming photographic element. The internal latent image-forming silver halide grains which received actinic radiation during imagewise exposure develop under these conditions at a slow rate as compared to the internal latent image-forming silver halide grains not imagewise exposed. The result is a direct-positive silver image. In color photography, the oxidized developer that is produced during silver development is used to produce a corresponding positive, direct-positive dye image. Multicolor direct-positive photographic images have been extensively investigated in connection with image transfer photography.
Direct-positive internal latent image-forming emulsions can take the form of halide-conversion type emulsions. Such emulsions are illustrated by Knott et al. U.S. Pat. No. 2,456,953 and Davey et al. U.S. Pat. No. 2,592,250.
More recently the art has found it advantageous to employ core-shell emulsions as direct-positive internal latent image-forming emulsions. An early teaching of core-shell emulsions is provided by Porter et al. U.S. Pat. No. 3,206,313, wherein a coarse grain monodispersed chemically sensitized emulsion is blended with a finer grain emulsion. The blended finer grains are Ostwald ripened onto the chemically sensitized larger grains. A shell is thereby formed around the coarse grains. The chemical sensitization of the coarse grains is "buried" by the shell within the resulting core-shell grains. Upon imagewise exposure latent image sites are formed at internal sensitization sites and are therefore also internally located. The primary function of the shell structure is to prevent access of the surface developer to the internal latent image sites, thereby permitting low minimum densities.
The chemical sensitization of the core emulsion can take a variety of forms. One technique is to sensitize the core emulsion chemically at its surface with conventional sensitizers, such as sulfur and gold. Atwell et al. U.S. Pat. No. 4,035,185 teaches that controlling the ratio of middle chalcogen to noble metal sensitizers employed for core sensitization can control the contrast produced by the core-shell emulsion. Another technique that can be employed is to incorporate a metal dopant, such as iridium, bismuth, or lead, in the core grains as they are formed.
The shell of the core-shell grains need not be formed by Ostwald ripening, as taught by Porter et al., but can be formed alternatively by direct precipitation onto the sensitized core grains. Evans U.S. Pat. Nos. 3,761,276, 3,850,637, and 3,923,513 teach that further increases in photographic speed can be realized if, after the core-shell grains are formed, they are surface chemically sensitized. Surface chemical sensitization is, however, limited to maintain a balance of surface and internal sensitivity favoring the formation of internal latent image sites.
It is generally well known in the photographic art to employ mixtures of negative-working emulsions to control the shape and position of the characteristic curve of a photographic element. Such practices are discussed by Zelikman and Levi, Making and Coating Photographic Emulsions, Focal Press, 1964, pp. 234 to 238. Blending of surface fogged direct-positive emulsions is also well known in the art, as illustrated by Smith and Illingsworth U.S. Pat. No. 3,615,573.
Whereas conventional negative-working emulsions and surface fogged direct-positive emulsions have been commonly prepared as either monodisperse or heterodisperse emulsions and blending of these emulsions has been undertaken, the characteristics of core-shell emulsions has dictated their preparation as monodisperse emulsions. For example, the Ostwald ripening process of Porter et al., cited above, requires that both the core and shell emulsions be monodisperse. Further, even when precipitation directly onto the core emulsion is undertaken, as described by Evans, cited above, monodisperse core emulsions permit control and uniformity of shell formation.
Blending of core-shell emulsions has been taught prior to this invention only when core-shell grains of similar average grain size have been blended. For example, Atwell et al., cited above, successfully blends monodisperse core-shell emulsions differing in the ratio of sulfur to gold internal sensitization. More recently monodisperse core-shell emulsions of the same average grain size, but with differing levels of surface chemical sensitization have been successfully blended.
Hoyen U.S. Ser. No. 320,902, filed Nov. 12, 1981 and commonly assigned, titled DIRECT-POSITIVE CORE-SHELL EMULSIONS AND PHOTOGRAPHIC ELEMENTS AND PROCESSES FOR THEIR USE, .Iadd.now U.S. Pat. No. 4,395,478, .Iaddend.discloses the use of polyvalent metal ion dopants in the shell of core-shell emulsions to reduce rereversal.
Evans et al. U.S. Ser. No. 320,891, filed Nov. 12, 1981 and commonly assigned, titled DIRECT REVERSAL EMULSIONS AND PHOTOGRAPHIC ELEMENTS USEFUL IN IMAGE TRANSFER FILM UNITS now abandoned in favor of continuation-in-part U.S. Ser. No. 431,912, filed Sept. 30, 1982, .Iadd.now abandoned in favor of U.S. Ser. No. 564,976, filed Dec. 23, 1983, issued as U.S. Pat. No. 4,504,570, .Iaddend.discloses image transfer film units containing tabular grain core-shell silver halide emulsions.
Black-and-white photography has relied traditionally upon developed silver to produce a viewable image. The silver that is not incorporated in the final image is frequently recovered, although in many applications, such as silver image transfer, for instance, silver is rarely recovered. Silver which forms the image is sometimes recovered, particularly from radiographic elements, but even in this instance the silver which remains in the element for imaging may be unavailable for reclamation for many years. Because of the cost of silver, it is highly desirable to make efficient use of it in photographic elements. One measure of the efficiency of silver use is covering power. Covering power is herein quantitatively defined as 100 times the ratio of maximum density to developed silver, expressed in grams per square decimenter. High covering power is recognized to be an advantaeous characteristic of black-and-white photographic elements. Covering power and conditions which affect it are discussed by James, Theory of the Photographic Process, 4th Ed., Macmillan, 1977, pp. 404, 489, and 490, and by Farnell and Soloman, "The Covering Power of Photographic Silver Deposits I. Chemical Development", The Journal of Photographic Science, Vol. 18, 1970, pp. 94-101.